Electric strip heater



Nov. 1, 1966 w. E. VELVEL 3,283,126

ELECTRIC STRIP HEATER Filed Dec. 10, 1965 2 Sheets-Sheet 1 FIG. 2

Fl G. 1

l, i l8 l9 I8 9 %Q m INVENTOR 4 WILLIAM EDWARD VELVEL ATTORNEY Nov. 1, 1966 w. E. VELVEL 3,283,126

ELECTRIC STRIP HEATER Filed Dec. 10, 1963 2 Sheets-Sheet 2 No v R INVENTOR WILLIAM EDWARD VELVEL BY fiy WM ATTORNEY United States Patent f 3,283,126 ELECTRIC STRIP HEATER William Edward Velvel, Claymont, Del., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware Filed Dec. 10, 1963, Ser. No. 329,490 9 Claims. (Cl. 219-388) This invention relates to electrical resistance strip heaters and to web treating machines embodying such heaters.

Electrical resistance strip heaters are widely used in a variety of industrial applications. Their primary use involves situations where localized heat in small quantities is needed. For example, strip heaters are used in crane cabs, ovens, warming ta bles, space heaters, and in various web treating machines.

An object of this invention is to provide a strip heater which heats, by conduction, a web or webs passing in contact with the strip. Another object is to provide a strip heater which can remain de-energized indefinitely at room temperature without consuming power, and after being energized can rapidly be heated to operating temperature.

Another object is to provide a strip heater that has a low heat capacity and is simple in construction and dependable in, use. A further object is to provide a strip heater capable of progressing rapidly through a succession of short hot and cold cycles. A still further object is to provide a strip heater that maintains a desired temperature within a narrow limit. A still further object is to provide a strip heater that is inexpensive, easy to manufacture and requires minimum maintenance. Still additional objects are to provide simple web treating machines embodying such strip heaters.

The strip heater of this invention comprises at least one thin, narrow, metal strip of high electrical resistance and low thermal capacity having depending sides that interfit with respective spaced grooves in the surface of an insulated support having an unheated area and a straight edge adjacent the strip. The preferred strip heaters also are provided with a means to accommodate thermal expansion and temperature control means.

The web treating machines of the invention comprise a drum or pressure roll transporting a web and coactive sheet into contact with the strip heater and has means for I guiding the web about the straight edge and separating the sheet from the web.

The invention will be further illustrated with reference to the accompanying drawings wherein the same reference numerals refer to the same parts throughout the several figures.

In the drawings:

FIG. 1 represents a top view of a strip heater which is spring loaded and has a temperature control means.

FIG. 2 represents a top view of a strip heater which is pre-stressed and shows the pre-stressing means.

FIG. 3 is a section along plane 33 of FIG. 1.

FIG. 4 is an enlarged elevation of the tractive clamp.

FIG. 5 is an enlarged cross-section of the strip heater along the line 3-3 of FIG. 1.

FIG. 5-A is a cross-section of an alternative heater, and

FIG. 6 is a diagram of a temptrature control circuit.

Referring now to FIGS. 1 and 5, the strip heater comprises a narrow strip 10 of metal having a high electrical 'resistivity having depending inwardly slanting sides 11 and 12 that interfit in respective grooves 13 and 14 in an insulated support 15. These grooves are so positioned that after the sides have been placed in the grooves, the

adjacent surface of the support is substantially a continuation of the surface of the strip. The unheated surface 16 of the support is narrow and terminates in an unheated straight edge 17 of the support. One end of the strip is anchored to the insulated support by a clamp 18 and screws 19, and the other end is gripped in a tractive clamp 20 between the base and plate 21 of the clamp. Screws 22 pass through the plate and threadably engage the base. This clamp is fitted with a threaded extension 23 which is free to move longitudinally within a fixed guide block 24 that is fastened to lateral frame member 25 by means or screws 26. The lateral frame is fastened to the base frame 27 in any suitable manner, e.g., bolts, screws or Weldments. A compression spring 28, located within a recess 29 in the guide block exerts a force against washer 30 and adjusting nut 31 which in turn causes a tensile force to be imposed upon the heated strip. When the strip 10 is connected to a source of electrical energy (not shown) its temperature rises and it tends to elongate due to thermal expansion. The expansion is absorbed by the spring 28 which causes the tractive clamp 20 to move. This movement is proportional to the temperature rise. When the temperature of the strip has reached the desired value, the threaded extension 23 of the clamp causes a movable leaf of a displacement microswitch 32 to move out of contact with a fixed contact, breaking the circuit and either terminating or reducing the input of electrical power to the heater strip.

This heater can then be mounted in a process machine with its long dimension at right angles to the direction of travel of a moving web 33 and lying essentially tangent to the plane of the passing web. The web is held in pressure contact against the strip by a pressure roll 34 to assure intimate contact and good heat transfer. This pressure roll is covered with a resilient material 35 which is a good thermal and electrical insulator and resists degradation at elevated temperatures, e.g., silicone rubber and tetrafiuoroethylene.

In operation, when the power supply is turned on, the strip is heated by the passage of electrical current. A web is put into motion by a drive means (not shown) and it passes between the pressure roll and the strip. The pressure roll presses down upon the moving Web and insures good contact between the heating strip and the web. This roll may be rotated either by the passing web or another driving means (not shown). The web is then heated by conduction and the desired thermal process takes place. During the operation, the thermal elongation of the strip is compensated by the spring loading or as will be explained later by pre-stressing the strip. The temperature can be regulated in the case of the spring loaded strip by a controller responsive to the displacement due to thermal expansion or in the case of a prestressed strip by a controller responsive to changes in tensile force. In either case, as will be explained later, the power input to the strip will be regulated to control its temperature.

In a process machine such as in a photothermographic copying machine, it may be desired to thermally treat a receptor sheet 36 in conjunction with the moving matrix web and later separating the web and sheet. This can be accomplished by subjecting the matrix web to a sharp change in direction by passing it around an unheated straight edge 17 of the insulated support. This sharp change of direction causes the matrix web to strip itself from the receptor, which is too stilf to follow the web around the straight edge.

In another embodiment, FIG. 2, instead of spring loading the heating strip to accommodate thermal expansion, it is pre-stressed. If the desired operating temperature of the strip is moderate, its total expansion per unit of length will not be excessive; and the heater can be prestretched at room temperature an amount greater than that expected at operating temperature without exceeding the elastic limit of the material. Under these conditions the length of the heater strip remains essentially constant as it heats from ambient to operating temperature, but the tensile stress in the strip decreases. If the linear expansion of the strip exceeds the amount of pre-stretch,

the tensile stress will drop to zero and expansion beyond this point will result in wrinkling and buckling. For example, a strip of Armco 1'7-7PH stainless steel 10 inches 'long x .25 inch wide x- .001 inch thick after being heat strip is forcibly elongated the desired amount between clamps 18 and 37 by rotating the tightening nut 39, on

the threaded extension of the tractive plunger that extends through a hole in a fixed lateral frame 40. The amount of stretch imposed upon the strip is monitored by the displacement indicator 41. After the desired stretch has been obtained, the clamp 37 is fastened by means of screws 42 and grips the insulated support and the extension of the strip can be severed near the clamp.

In the alternative construction in FIG. 5-A, the strip heater is in single enclosed groove in the insulated support, the outer side having a straight edge for the unit which .can be in an offset ledge in the support as shown, or

extending along the side wall with no offset, or there can be only one depending edge that interfits with a recessed area in the support. I

FIG. 6 indicates a suitable electrical circuit for a preferred embodiment of the strip heater and its temperature control means.

a 10:1 stepdown transformer T. The electrical power induced in the secondary then flows into the strip heater R-2. After the strip temperature rises and the strip elongates to the set point, the microswitch 32 in FIG. 1 opens. When this occurs the power flows through variable resistor R-1 and the input into the heating strip is reduced.

" In many applications of this invention a stepdown transformer is not necessary, and if the variable resistor R-].

is not used, instead of the microswitch reducing the power input it would terminate the input. After the power had i been terminated, the strip would contract because of the fall in temperature thereupon closing the microswitch and re-establishing the power flow to bring the temperature of the strip again to the desired level.

In the preferred embodiment the pressure roll is used to insure intimate contact between the moving web, receptor sheet if any, and the strip heater. This roll can be driven by drive means or can be an idler with rotation supplied by the moving web. The roll can be mounted so that whenever the pressure is needed, mechanical means can move the roll from an inactive position into pressure contact with the strip.

As the strip is electrically charged and operated at increased temperatures, the roll should have a surface that is resilient and a good thermal and electrical insulator.

. A silicone rubber covering may be used. It is obvious that in some applications it will not be necessary to use a Q roll, and that a pressure pad with a resilient, insulated surface may be used.

For practical reasons the electrical resistance strip element should have an electrical resistance of around 100 microhm cm., and a low thermal mass so it can heat and cool quickly. The material used should have a high elastic limit, should be abrasion resistant to resist the frictional action of the moving web and rigid enough to withstand pressure loading. The resistance element can be a nickelchromium alloy, a nickel-chromium, iron alloy, or a iron-chromium-aluminum alloy.

SW is the main power switch and upon turn-on the power flows through a microswitch MS into strip are possible.

A satisfactory element is a 10" x .25 x .001" strip of Armco l77PH stainless steel that has been heat treated to raise its yield point to over 200,000 lbs. per square inch. This Armco l7-7PH stainless steel is an alloy of steel with 0.09% carbon, 1.0% manganese, 0.04% phosphorous, 0.03% sulfur, 1.0% silicon, l6.0l8.0% chromium, 6.5 to 7.73% nickel, and 0.75-l.5% aluminum. After the alloy is formed it is heat treated by a three step process. First, it is heated to 1750 F. (il5 F.) and held for 10 minutes at that temperature and then air cooled to room temperature. Second, it is cooled to F. (:10 F.) and held for 8 hours at that temperature. The last step is heating it to 950 F. (il0 F.) holding it at that temperature for 60 minutes and then air cooling to room temperature. This material then offers the proper combination of the following properties; corrosion resistance, linear relationship between stress and strain, high elastic limit, fabricability, hardenability, acceptable electrical resistivity, and availability in a useable condition from stock.

The strip should be dovetailed while in the annealed state, and this dovetailing enables the strip to cling to its support despite the tendency of the passing web to dislodge or distort it by pressure and frictional drag. Dovetailed angles of 135 and were found to be satisfactory though many other combinations of angles are possible and a side of the strip can be bent in other shapes, such as arcs.

The support for the strip must be a good thermal and electrical insulator and have the ability to resist deforma tion at elevated temperatures. The support must also be hard enough to maintain a sharp straight edge that can be used for stripping. The support can be made from the following materials: silicone-fiberglass, a tetrafluoroethylene resin-fiberglass; some of the ceramic insulators such as porcelain and lava; or Pyrex made by Corning Glass Works, Corning, New York. The straight edge of the support must be within close proximity to the heated strip to provide the sharp change in direction needed for stripping and in addition allow the stripping to take place at a cooler place. If the web were stripped on the edge of the heater, it would cause an overheating of the web and a resultant failure in tension, as-the edges of the strip become superheated during operation.

Various means of controlling the temperature of the There can be self-regulation of temperature as a function of the thermal, lineal expansion or as a function of the change in tensile stress. Various means can be used to terminate or reduce the power input once the required temperature has been reached and reestablish or increase power input upon loss of thermal energy by the strip.

Various systems that would sense a change in displacement may be used with the spring loaded embodiment. The elongation of the strip could be mechanically transmitted to an iron core in the magnetic field of a differential transformer thereby inducing a voltage change in the secondary of the transformer. This voltage change is proportional to the increase in length. The differential transformer could be connected with a means that would amplify and use this voltage change to terminate or reduce the electrical power input to the strip once the desired temperature had been reached. A displacement switch that can detect minute mechanical displacement could also be used. This device would sense the displacement of the spring-loaded end of the heated strip and once the set elongation is reached the switch would terminate or reduce the power input One such switch is Model BZR manufactured by Microswitch, Freeport, Illinois, and which is reported to be capable of detecting a displacement differential of .0003".

In the embodiment where the strip is pre-stressed the temperature can be regulated by sensing the total tensile force in the strip. The measurement can be accomplished by using a material that responds with large changes in electrical resistance when it is acted upon by pressure. For example, carbon granules as in a telephone transmitter, or a pressure-sensitive paint containing certain combinations of the lanthanide rare earth series mixed with zirconium tetrachloride such as that manufactured by Clark Electronic Laboratories, Palm Springs, California, or a solid state, wafer type resistance transducer, Model CSl-55 also manufactured by Clark Electronic Laboratories could be used to transduce the pressure in the strip into electrical current.

In application the pressure-sensitive electrically-conductive member would be inserted into the heater structure in place of spring 28 in FIGURE 1, subjecting it to a compressive force, the magnitude of which is determined largely by the heater strip and the degree to which it is forcibly elongated. The pressure-sensitive element would exhibit a certain electrical conductivity related to the force imposed upon it. When the heater strip is energized it will tend to elongate due to thermal expansion. Inasmuch as it has already been forcibly elongated in tension, its effective length will remain essentially unchanged, but the tensile force within it will decrease. This will reduce the force sensed by the pressure-sensitive element, changing its electrical conductivity. The change can be used to reduce, terminate or otherwise control power input to the heater strip as already described.

The strip heater of this invention is useful for many applications where a moving web must be thermally treated. The strip heater is particularly useful in ofiice copy machines of the thermal transfer type in which an image of an original is produced on a receptor sheet by performing a thermal transfer operation from a selectively photopolymerized matrix material. A machine of this type is disclosed in assignees copending patent ap plications Heiart and Velvel, Ser. No. 234,616 filed Nov. 1, 1962, US. Patent 3,211,074, Oct. 12, 1965, and Cohen, Ser. No. 250,856 filed Jan. -11, 1963. In the machine of the Heiart and Velvel application, the heated roll, the stripping edge and the pressure roll would be replaced by the heater of this invention. In the machine of the Cohen application this invention would replace the heating roll and the pressure roll.

Suitable photopolymerizable matrix materials for use in such machines are disclosed in Burg and Cohen US. Patent 3,060,023 issued Oct. 23, 1962, and US. patent application Burg and Cohen, Ser. No. 163,078 filed Dec. 29, 1961, US. Patent 3,218,167, Nov. 16, 1965; Heiart, Ser. No. 123,651 filed July 13, 1961, U8. Patent 3,202,508, Aug. 24, 1965; Cohen and Luebbe, Ser. No. 156,518 filed Dec. 1, 1961, US. Patent 3,198,633, Aug. 3, 1965; and Burg, Ser. No. 234,214 filed Oct. 30, 1962, US. Patent 3,203,805, Aug. 31, 1965.

The machine of this invention is very useful in a photopolymer copying process. It is useful in other photothermographic copying processes, such as the vesicular diazo process, to develop the image in the photosensitive material after exposure to actinic light. The strip heater can also find application in a machine using a thermallydevelopable silver-halide type of photosensitive material containing the components of a combined developer and fixer plus bound water that is released upon heating.

The strip heater can be used in non-photographic webprocessing devices where the web is a nonconductor, e.g., for continuously laminating or heat sealing several moving webs together as they pass in pressure contact with the heating strip. Also, it could be used to delaminate or strip apart two Webs cemented with a thermoplastic by passing the sandwich over the heater bar and drawing one of the webs around the unheated straight edge.

The invention could find application where the following characteristics, singly or in combination, are required: the ability to stand by at ambient temperatures indefinitely without consuming power and reach operating temperatures up to 200 C. within one or two seconds after being energized; the ability to heat a web or webs passing in contact with the heater; with simultaneous application of pressure if required; and the ability to withstand pressure loading, frictional drag and abrasion of web material passing in contact with the heater.

This invention has many advantages as it provides a simple, inexpensive, quick-heating machine component that can be used for thermally processing moving webs. It is structurally rigid and durable and operates with a minimum power requirement.

The strip heater being essentially straight and planar over its entire length can be placed substantially in contact with the entire width of an integral strip of a continuous web or sheet of material being drawn past the heater. Heat is generated quickly and uniformly over the effective length and width of the strip heater. By placing two or more strip heaters parallel to each other with the edges in parallel grooves, a wider heating surface can be attained. Individual heating controls can be provided for each strip.

A further advantage of the strip heater of this invention is that a significant temperature rise can be attained with out wrinkling or buckling of the strip. Further advantages are that the strip heater can withstand pressure loading, elevated temperatures and abrasive frictional drag.

I claim:

1. An electrically energizable strip heater unit for heating a moving web comprising (a) la. long, narrow, thin metal conductor strip of high electrical resistivity, high elastic limit capable of thermal elongation and having depending lengthwise sides;

( b) a thermally and electrically insulating support for said strip having a recessed area or areas of small cross-section extending downwardly from the surface of the support and interfitting with a depending side or sides of the strip; the heater unit being further characterized by having (1) the bottom surface of the strip being in engagement with the top surface of the support;

(2) the strip being connected to a source of electrical energy so that the strip will be heated by the passage of electrical current;

(3) the support and strip being disposed adjacent the path of the web;

(4) the lengthwise axis of the strip extending across the path of the web and at right angles to the axis of the web travel;

(5) one end of the strip being fastened to the support;

(6) a lengthwise edge of the strip or of an adjacent surface of the support, downstream from the path of the web, serving as a straight edge for guiding a web as it passes from the unit at an angle from the surface of the support; and

(7) the adjoining upper surfaces of the strip and the support being in substantially the same plane.

2. A strip heater unit according to claim 1 wherein at least one of the depending sides of the strip depends at an acute angle to the outer surface of the strip and interfits with a groove in the insulating support.

3. A strip heater unit according to claim 1 wherein the other end of the conductor strip is connected to a tensioning spring that is connected to the support so that the strip is under tension.

4. A strip heater unit according to claim 1 having means to sense the thermal elongation of the strip and to reduce or terminate the electrical energy supplied to the strip to prevent its temperature from exceeding a desired operating temperature.

5. A web treating machine comprising: (A) an electrically energizable strip heater unit for heating a moving web comprising (a) a long, narrow, thin metal conductor strip of high electrical resistivity, high elastic limit capable of thermal elongation and having depending lengthwise sides;

(b) a thermally and electrically insulating support for said strip having a recessed area or areas of small cross-section extending downwardly from the surface of the suppont and interfitting with a depending side or sides of the strip; the heater unit being further characterized by having (1) the bottom surface of the strip being in engagement with the top surface of the support;

(2) the strip :being connected to a source of electric-a1 energy so that the strip will be heated by the passage of electrical current;

(3) the support and strip being disposed adjacent the path of the Web;

(4) the lengthwise axis of the strip extending across the path of the web and at right angles to the axis of the Web travel;

(5) one end of the strip being fastened to the support;

(6) a lengthwise edge of the strip or of an adjacent surface of the support, downstream from the path of the web, serving as a straight edge for guiding a web as it passes from the unit at an angle from the surface of the support; and

(7) the adjoining upper surfaces of the strip and the support being in substantially the same plane, and

(B) a pressure memberadjacent said strip for guiding and pressing a web into contact with the conductor strip. 6. A machine according to claim 5 wherein said pres- 5 sure member is a pressure roll.

7. A machine according to claim 5 wherein at least one of the depending sides of the strip depends at an acute angle to the outer surface of the strip and interfits with a groove in the insulating support.

8. A machine according to claim 5 wherein the other end of the conductor strip is connected to a tensioning spring that is connected to the support so that the strip is under tension.

9. A machine according to claim 4 having means to sense the thermal elongation of the strip and to reduce or terminate the electrical energy supplied to the strip to prevent its temperature from exceeding a desired operating temperature.

References Cited by the Examiner UNITED STATES PATENTS 224,612 2/1880 Sawyer 21950 1,506,812 9/1924 Brown 338316 X 1,661,026 2/1928 Asch 1325 1,923,644 8/1933 Simpson 338-316 X 1,949,450 3/1934 Brown 219455 2,450,362 9/1948 Scott 219-l56 2,796,913 6/1957 Fener et al. 156251 3,028,294- 4/1962 Histed 156583 X FOREIGN PATENTS 420,569 12/ 1934 Great Britain. 915,782 1/ 1963 Great Britain.

RICHARD M. WOOD, Primary Examiner.

C. L. ALBRITTON, Assistant Examiner. 

1. AN ELECTRICALLY ENERGIZABLE STRIP HEATER UNIT FOR HEATING A MOVING WEB COMPRISING (A) A LONG, NARROW, THIN METAL CONDUCTOR STRIP OF HIGH ELECTRICAL RESISTIVITY, HIGH ELASTIC LIMIT CAPABLE OF THERMAL ELONGATION AND HAVING DEPENDING LENGTHWISE SIDES; (B) A THERMALLY AND ELECTRICALLY INSULATING SUPPORT FOR SAID STRIP HAVING A RECESSED AREA OR AREAS OF SMALL CROSS-SECTION EXTENDING DOWNWARDLY FROM THE SURFACE OF THE SUPPORT AND INTERFITTING WITH A DEPENDING SIDE OR SIDES OF THE STRIP; THE HEATER UNIT BEING FURTHER CHARACTERIZED BY HAVING (1) THE BOTTOM SURFACE OF THE STRIP BEING IN ENGAGEMENT WITH THE TOP SURFACE OF THE SUPPORT; (2) THE STRIP BEING CONNECTED TO A SOURCE OF ELECTRICAL ENERGY SO THAT THE STRIP WILL BE HEATED BY THE PASSAGE OF ELECTRICAL CURRENT; (3) THE SUPPORT AND STRIP BEING DISPOSED ADJACENT THE PATH OF THE WEB; (4) THE LENGTHWISE AXIS OF THE STRIP EXTENDING ACROSS THE PATH OF THE WEB AND AT RIGHT ANGLES TO THE AXIS OF THE WEB TRAVEL; (5) ONE END OF THE STRIP BEING FASTENED TO THE SUPPORT; (6) A LENGTHWISE EDGE OF THE STRIP OR OF AN ADJACENT SURFACE OF THE SUPPORT, DOWNSTREAM FROM THE PATH OF THE WEB, SERVING AS A STRAIGHT EDGE FOR GUIDING A WEB AS IT PASSES FROM THE UNIT AT AN ANGLE FROM THE SURFACE OF THE SUPPORT; AND (7) THE ADJOINING UPPER SURFACES OF THE STRIP AND THE SUPPORT BEING IN SUBSTANTIALLY THE SAME PLANE. 